Broadly speaking, the developments to be described below relate to communications systems in which a transmitter may, at a given time, use any of a set of independent parallel communication channels to transmit messages to a particular receiver. A schematic view of such a communication system is shown in FIG. 1, including transmitter 10, channels 21, 22, and 23, and receiver 30.
A typical example of such a system is a shared packet data communication system in which the transmitter communicates simultaneously with several receivers using a set of N communications channels that are shared among the different receivers. Parallel channels are used in such a system to enable the transmitter to transmit to more than one receiver at any time instant. Each receiver is pre-configured to be able to receive signals from only M out of the N shared channels. The set of M channels that a receiver monitors is pre-defined, and may be different for different receivers. The transmitter schedules up to N users at a time and transmits messages to each of them via a different channel chosen from the N available channels.
For instance, suppose that the transmitter communicates with 20 different receivers, and has access to N=4 parallel shared channels. Suppose also that each receiver is pre-configured to listen to one of M=2 channels, where the specific set of 2 channels from among the 4 channels that a particular receiver monitors is also pre-defined. Then, during any transmission time, the transmitter may choose to transmit information to a set of K receivers of its choice from among the 20 available receivers, where K≦4. The transmitter would use K communication channels for these transmissions, one for each receiver, such that the channel used for each user belongs to the set of channels that that user monitors.
HSDPA is a high-speed packet data transmission system for the downlink, i.e., the link from the base station to the mobile station, in a wireless communication system. The current implementation of HSDPA is defined in Release 5 of the UMTS specification published by the 3rd Generation Partnership Project (3GPP).
In HSDPA, a group of users is scheduled in each transmission time interval (TTI), which is 2 ms long. In other words, within the 2-ms duration of a single TTI, a scheduler in the base station selects a small number (typically 0 to 8) of users to which data is transmitted in that 2-ms interval. Data is transmitted to each of the scheduled users via a physical channel called the HS-PDSCH, and the corresponding control information is transmitted to each scheduled user on a shared control channel called the HS-SCCH.
HS-SCCH transmissions to the scheduled users in a TTI is an example of the shared channel communication system described above. The base station transmitter has a set of N HS-SCCH channels available, and each user is capable of monitoring up to M=4 HS-SCCH channels. If K users are scheduled in a TTI, then the base station must transmit HS-SCCH information to each of the K scheduled users using a different HS-SCCH channel, such that the HS-SCCH channel used for each user belongs to the set of channels monitored by the user.
The HS-SCCH carries several parameters that are required by the user to decode the corresponding HS-PDSCH transmission. These may include, e.g., parameters that identify the set of codes over which HS-PDSCH signals are transmitted for that user. HS-PDSCH codes are OVSF spreading codes at spreading factor 16, of which there are a total of 16 codes. A subset of these 16 codes are available for use by HS-PDSCH at the base station. According to the current specification of the HS-SCCH channel, it carries information that determines exactly the set of one or more spreading codes from among the available HS-PDSCH codes over which data is transmitted to the scheduled user.